WINTER TRANSPIRATION
PURPOSE:
To demonstrate the relative amounts of water lost by plants through transpiration.
One or more geranium plants, freshly cut branches from a winter evergreen tree and deciduous tree, hand lenses, 3 or more quart-size plastic bags with twist ties, two or more 250 ml flasks with water, light source.
INTRODUCTION:
Transpiration is the loss of water from plant tissues by evaporation. Most transpiration takes place through tine openings called stomates located on the surfaces of leaves. If you look closely at the needles of pine, spruce or hemlock, you will see rows of stomates appearing as white lines. To see the actual stomates you would have to use a microscope. The broad leaves found on trees, shrubs and herbs also have many microscopic stomates through which gases are exchanges. A pair of guard cells surrounding each stomate controls the size of the opening. The outside layer of cells on the top and bottom of a leaf, the epidermis, is protected by a layer of waxy material called the cuticle. It prevents excess water loss. If you look closely at the stems of many woody plants you will see small structures called lenticels that look like dots or lines. Like stomates, they allow gases to pass into and out of the plant. Transpiration occurs when water vapor, an invisible gas, is lost through the stomates, lenticels and cuticle. In winter, deciduous trees shed their leaves, but evergreen trees do not. Does transpiration occur in plants during the winter? In this activity you will set up an experiment to find out.
Use what you know about plant structure and transpiration to make some predictions about transpiration in house plants, and in winter evergreen and deciduous plants.
1. What evidence do you expect to find if transpiration occurs from any of those three plants?
2. Which plant do you think will give off the greatest amount of water vapor? Which will give off the least amount of water vapor? Why do you think so?
PROCEDURE:
A. Select a leafy geranium plant that has been kept inside in a flower pot. Put one plastic bag over a leafy twig. Inflate the bag with room air; do not breath into the bag. Seal the bag tightly around the stem with a twist tie.
B. Put one plastic bag over a freshly cut evergreen twig with needles, such as pine, spruce or hemlock. Put another plastic bag over a freshly cut leafless deciduous twig such as maple, beech or ash. Inflate each bag with room air and close tightly with a twist tie. Put the cut end of each twig into a flask containing water.
C. Use a hand lens to look for rows of stomates on the underside of the evergreen needles. Look for lenticels on the stems of all three plants.
D. Expose all three twigs to light for 24 hours. Then look for evidence of transpiration. Record your observations on the back of this sheet.
CONCLUSIONS:
3. Did you find any evidence that transpiration occurred? If so, describe it.
4. Which plant gave off the greatest amount of water vapor? Which gave off the least? Were your predictions correct?
5. Which leaf and stem structures are involved in transpiration? DISCUSSION:
6. How do you account for any differences in the amount of water given off by the geranium plant compared to the evergreen plant?
7. How do you account for any differences in the amount of water give off by the geranium plant compared to the deciduous plant?
8. What results would you expect to get if you tested an evergreen twig or a leafy deciduous twig collected during the summer?
9. Why did the invisible water vapor given off by the plant become visible in the plastic bags?
10. How does the use of the lamp increase the amount of water vapor given off by plants?
11. During the winter, deciduous trees in our area are dormant (inactive). Based on the results of this experiment, do you think winter evergreen trees are active like the geranium or dormant like deciduous trees? Explain.
12. Do you think winter evergreen trees carry on photosynthesis during the winter? Explain your reasoning.